Rework SubConfig so it doesn't share mutable state (#2916)

mkdocs/config/config_options.py

@@ -49,19 +49,17 @@ class BaseConfigOption:
         """
 
 
-class SubConfig(BaseConfigOption, Config):
+class SubConfig(BaseConfigOption):
     def __init__(self, *config_options):
-        BaseConfigOption.__init__(self)
-        Config.__init__(self, config_options)
+        super().__init__()
         self.default = {}
-
-    def validate(self, value):
-        self.load_dict(value)
-        return self.run_validation(value)
+        self.config_options = config_options
 
     def run_validation(self, value):
-        Config.validate(self)
-        return self
+        config = Config(self.config_options)
+        config.load_dict(value)
+        config.validate()
+        return config
 
 
 class ConfigItems(BaseConfigOption):

mkdocs/tests/config/config_tests.py

@@ -283,6 +283,26 @@ class ConfigTests(unittest.TestCase):
             self.assertEqual(len(errors), 1)
             self.assertEqual(warnings, [])
 
+    SUBCONFIG_TEST_SCHEMA = {
+        "items": mkdocs.config.config_options.ConfigItems(
+            ("value", mkdocs.config.config_options.Type(str)),
+        ),
+    }.items()
+
+    def test_subconfig_with_multiple_items(self):
+        # This had a bug where subsequent items would get merged into the same dict.
+        conf = config.Config(schema=self.SUBCONFIG_TEST_SCHEMA)
+        conf.load_dict(
+            {
+                'items': [
+                    {'value': 'a'},
+                    {'value': 'b'},
+                ]
+            }
+        )
+        conf.validate()
+        self.assertEqual(conf['items'], [{'value': 'a'}, {'value': 'b'}])
+
     def test_multiple_markdown_config_instances(self):
         # This had a bug where an extension config would persist to separate
         # config instances that didn't specify extensions.